Power companies furnish alternating-current (“AC”) power in the form of supply voltages that vary largely sinusoidally with time. Referring to FIG. 1, it illustrates a simplified version of a power-supply circuit in which AC power supply 20, such as that of a power company, provides analog input AC supply voltage VAC at fundamental power supply frequency fAC to load 22 at a consumer's location. AC supply voltage fAC is specifically given as:VAC=V0 sin(ωACt)  (1)where V0 is a voltage amplitude component, ωAC is the fundamental angular supply frequency equal to 2πfAC, and t is time. Load current ILD flows through load 22, causing it to consume instantaneous power P1 equal to ILDVAC.
The time variation of load current ILD depends on the constituency of load 22. Resistors, inductors, and capacitors, all of which are linear electronic elements in their ideals forms, may be variously present in load 22. Waveforms for supply voltage VAC, load current ILD, and instantaneous power P1 for a full VAC cycle are presented in FIGS. 2a-2c for three different linear implementations of load 22.
Load current ILD ideally varies in a sinusoidal manner fully in phase with supply voltage VAC. This situation when arises when load 22 is purely resistive as represented by the waveforms shown in FIG. 2a. The phase angle φ between the ILD and VAC waveforms is zero. Instantaneous power P1 varies in a sinusoidal manner at twice supply frequency fAC. When load 22 is purely resistive, it consumes all the power available from power supply 20. The average consumed power PAV is therefore the maximum possible.
If load 22 is purely inductive as represented by the waveforms depicted in FIG. 2b, phase angle φ is 90°. As a result, average consumed power PAV is zero. All of the power provided by power supply 20 is returned to it. The same arises when load 22 is purely capacitive except that phase angle φ is −90°.
When load 22 consists of a combination of resistive, inductive, and capacitive elements, consumed power PAV lies between zero and the maximum possible (except in the rare instances where the effects of capacitive and inductive elements identically cancel each other). Part of the power provided by power supply 20 is returned to it. This situation is illustrated by the waveforms shown in FIG. 2c for an implementation of load 22 as a resistive/inductive combination. Load current ILD is then generally given as:ILD=I0 sin(ωACt+φ)  (2)where I0 is a current amplitude component, and phase angle φ is between −90° and 90°.
The efficiency of power consumption is characterized in term of power factors. The power factor PFPhase for a phase-shifted implementation of load 22 as a linear combination of resistive, inductive, and capacitive elements is given as:
                              PF          Phase                =                                            ∫              0                              I                F                                      ⁢                                          I                0                            ⁢                              sin                ⁡                                  (                                                                                    ω                                                  A                          ⁢                                                                                                          ⁢                          C                                                                    ⁢                      t                                        +                    φ                                    )                                            ⁢                              V                0                            ⁢                              sin                ⁡                                  (                                                            ω                                              A                        ⁢                                                                                                  ⁢                        C                                                              ⁢                    t                                    )                                            ⁢                                                          ⁢                              ⅆ                t                                                                        I              LDRMS                        ⁢                          V              LDRMS                        ⁢                          t              P                                                          (        3        )            where ILDRMS is the root-mean-square (“RMS”) value of load current ILD, VACRMS is the RMS value of supply voltage VAC, and tF is the period of time, at least one cycle, over which power factor PFPhase is determined. Phase-shifted power factor PFPhase is one, the maximum possible, when phase angle φ is 0°, and zero when phase angle φ is ±90°. It is generally desirable that load 22 be configured to make power factor PFPhase as close to one as possible.
Load 22 may also include non-linear elements which cause load current ILD to have overtones of fundamental supply frequency fAC. Each overtone frequency fm is given as:fm=mfAC  (4)where m, a positive integer, is the overtone number. With the fundamental frequency constituent ILD0 in load current ILD being given as:ILDO=I0 sin(ωACt)  (5)each overtone frequency constituent ILDm in current ILD at zero phase angle is given as:ILDm=Im sin [(m+1)ωACt]  (6)where Im is a positive current amplitude component for the mth overtone constituent ILDm. For the overtone case, load current ILD is then given generally as:
                              I          LD                =                                            I              0                        ⁢                          sin              ⁡                              (                                                      ω                                          A                      ⁢                                                                                          ⁢                      C                                                        ⁢                  t                                )                                              +                                    ∑                              m                =                1                            M                        ⁢                                          I                m                            ⁢                              sin                ⁡                                  [                                                            (                                              m                        +                        1                                            )                                        ⁢                                          ω                                              A                        ⁢                                                                                                  ⁢                        C                                                              ⁢                    t                                    ]                                                                                        (        7        )            where M, the number of overtones, can theoretically go to infinity. More generally, Eq. 7 includes a summation of overtone cosine terms to accommodate phase angle φ.
FIG. 3 illustrates a prior art example of VAC and ILD waveforms for a situation in which load 22 contains non-linear elements. FIG. 4 depicts the amplitude of each overtone current constituent ILDm relative to the amplitude of fundamental current constituent ILD0 for this example. The relatives amplitudes of the odd-numbered overtones are small here due to the substantial ILD symmetry about the VAC peak values.
Part of the power in each overtone current constituent ILDm is returned to power supply 20. The power factor PFOvertone for a non-linear implementation of load 22 is given as:
                              PF          Overtone                =                              P                          AV              ⁢                                                          ⁢              0                                                          P                              AV                ⁢                                                                  ⁢                0                                      +                                          ∑                                  m                  =                  1                                M                            ⁢                              P                                  AV                  ⁢                                                                          ⁢                  m                                                                                        (        8        )            where PAV0 is the average power associated with fundamental frequency constituent ILD0, and PAVm is the average power associated with each overtone frequency constituent ILDm. As with phase-shifted power factor PFPhase, it is generally desirable that load 22 be configured to make overtone power factor PFOvertone as close to one as possible. That is, load 22 is preferably configured to minimize the presence of overtones in load current ILD.
Load 22 typically includes equipment which converts the AC power into direct current (“DC”) power for use by DC equipment. FIG. 5 depicts a conventional example in which load 22 consists of common-mode transformer 24, bridge rectifier 26, and DC load 28. Bridge rectifier 26, which is formed with four pn diodes DA, DB, DC, and DD, performs full-wave rectification on AC supply voltage VAC to produce analog full-wave rectified voltage VFWR provided to DC load 28. FIG. 6 illustrates how full-wave rectified voltage VFWR typically appears relative to supply voltage VAC. Subject to the full-wave rectification, all of the preceding power considerations dealing with supply voltage VAC and load current ILD substantially apply to rectified voltage VFWR and the corresponding DC load current flowing through DC load 28.
Power factor correction circuitry is commonly incorporated into load 22 for the purpose of increasing power factors PFPhase and PFOvertone. FIG. 7 illustrates a power-supply circuit containing switch-mode power factor correction circuitry as described in Acatrinei, U.S. Patent Publication 2005/0212501 A1. Load 22 in FIG. 7 consists of low-pass filter 30, bridge rectifier 26, constant pulse proportional current power factor correction inverter circuit (“CPPC-PFC-IC”) 32, constant pulse generator 34, and further DC load 36. Low-pass filter 30 is formed with common-mode transformer 24 and capacitors CA and CB. CPPC PFC-IC 32 consists of capacitor CC, inductor LA, diode DE, and power switching n-channel field-effect transistor QA.
Acatrinei's power factor correction circuitry operates as follows. Constant-pulse generator 34 operates at a fixed duty cycle to provide power transistor QA with switch voltage VSW as a sequence of pulses of fixed pulse width tW at fixed pulse frequency fP much greater than fundamental supply frequency fAC. This causes transistor QA to alternately turn on and off many times during each VFWR wave. Switch current ISW flows through transistor QA in response to each VSW pulse and drops rapidly to zero when each pulse ends.
The voltage across an inductor is the product of its inductance and the time rate of change of the current through the inductor. The change ΔIL in current IL through inductor LA is thereby approximately given by:
                              Δ          ⁢                                          ⁢                      I            L                          =                              (                                          V                L                                            L                L                                      )                    ⁢                      t            W                                              (        9        )            where VL is the voltage across inductor LA, and LL is its inductance. Inductor current IL is the sum of switch current ISW and current ID through diode DE.
Total ground current IT substantially equals inductor current IL which, in turn, substantially equals the full-wave rectified version of load current ILD. Letting IDAV, ISWAV, and ITAV be the respective average values of diode current ID, switch current ISW, and total current IT during a VSW pulse, the result of Eq. 8 is that instantaneous voltages VAC and VFWR and average currents IDAV, ISWAV, and ITAV for Acatrinei's power factor correction circuitry typically have the waveform shapes shown in FIG. 8a for a VFWR cycle. FIG. 8b illustrates how instantaneous currents ID, ISW, and IT change with switch voltage VSW during brief time portion 38 in FIG. 8a. 
Inasmuch as total ground current IT substantially equals the full-wave rectified version of load current ILD, average total ground current ITAV in Acatrinei's power factor correction circuitry should closely approach a sinusoidal shape during each VFWR cycle in order to make overtone power factor PFOvertone close to one. As indicated in FIG. 8a, average switch current ISWAV is of nearly sinusoidal shape. However, average total current ITAV is closer to a triangular shape than to a sinusoidal shape. Average total current ITAV thus includes a significant overtone constituency, causing power factor PFOvertone to be significantly below one.
It would be desirable to have switch-mode power factor correction circuitry in which the rectified overall load current is of nearly sinusoidal shape during each wave of the rectified AC supply voltage.